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Protostomes are a nephrozoan taxon of animals above the level of phylum. It is not part of the normal system of classification.

They are a subtaxon of the Bilateria, and are contrasted with the deuterostomes. Protosomes are distingued by their embryo: in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth.

DNA sequence analysis suggests that protostome animals can be divided into three major groups:

• Ecdysozoa, e.g. arthropods, nematodes
• Platyzoa, e.g. platyhelminthes, rotifers
• Lophotrochozoa, e.g. molluscs, annelids

as well as a number of minor taxa, like the Chaetognatha.